The genetic legacy of more than a century of stocking trout: a case study in Rocky Mountain National Park, Colorado, USA

Abstract

Human introductions can obscure the diversity and distribution of native biota; hybridization with and replacement by introduced congeners is a primary conservation threat, particularly in salmonids. Cutthroat trout (Oncorhynchus clarkii) are an important component of biodiversity in the American West, and all recognized subspecies are targets for state and federal conservation efforts. Rocky Mountain National Park (RMNP) in northern Colorado is a microcosm of trout introductions that happened worldwide. We used a combination of extensive stocking records and molecular genetic data to ask whether native trout populations persist despite stocking and whether patterns in the distribution of cutthroat trout clades could be explained by source and intensity of stocking. Nearly 15 million cutthroat trout were stocked into RMNP from a mosaic of sources in the 20th century. A single lineage of cutthroat trout was historically native to each side of the Continental Divide in RMNP, but we detected at least five divergent clades of cutthroat trout in 34 localities on both sides of the Divide. The distribution of lineages was predicted by stocking pressure and source but not by which lineage was historically native. The future of mixed and non-native cutthroat trout populations in RMNP poses a substantial conservation challenge.